In the study of the branched-chain amino acids, the initial step in the catabolism of leucine is catalyzed by leucine 2,3-aminomutase. The product, beta-leucine, is deaminated and cleaved to give acetate and isobutyrate. Leucine 2,3-aminomutase is dependent upon coenzyme-beta and is stimulated by FAD, coenzyme A, DPN, pyridoxal phosphate. The enzyme is present in clostridia, mammalian liver, human tissue, and plant material. Regardless of the source, the enzyme is linked to cobalamin. A gas chromatographic assay of the branched-chain amino acids has been developed.